G C S E P h y s i c s – A t o m i c S t r u c t u r e

( P a g e 1 )

Key words: 1. Atoms - an atom has no overall charge 2. Mass number = protons + neutrons 3. Atomic number = protons (also = electrons) 4. Isotopes – atoms of the same element with the same numbers

of protons and electrons but different numbers of neutrons. 5. Background radiation – low level radiation present at all

times. 6. Irradiation – exposing an object to a radioactive source. 7. Contamination – unwanted radioactive atoms on an object. 8. Half-life – the time taken for the number of radioactive

nuclei in a sample to halve.

So in carbon: Protons = 6 Electrons = 6 (same as protons) Neutrons = 12 – 6 = 6

Early ideas; Before the

discovery of the electron atoms were

tiny spheres, they couldn’t

be divided

Alpha particle scattering; Showed that the mass of an atom was concentrated in the centre, it was charged too

Niels Bohr; Adapted the nuclear model

suggesting electrons in orbitals at set distance

Nucleus development; Experiments now show

nucleus is made of smaller particles of

positive charge

James Chadwick; Evidence to show the existence of neutrons

in the nucleus

Development of the model of the atom;

New experimental evidence may lead to the model being changed or replaced.

Plum pudding; After the electron was discovered the atom became a ball of positive charge with negative electrons scattered in it

Isotopes; • Atoms that have

the same number of protons (element), but different numbers of neutrons

Nuclear Model Plum Pudding

model

Where is the

mass?

Concentrated in the

centre/nucleus

Evenly spread out

/ distributed

Where is the

positive

charge?

Positive charge

occupies a small part

in the centre of the

atom

Positive charge

spread

throughout the

atom

Where are

the electrons?

Electrons orbit some

distance from the

centre / electrons in

shells

Electrons

embedded in the

(mass) of positive

(charge)

How much

space is

there?

The atom is mainly

empty space

Solid mass

Comparing the nuclear model to the plum pudding model

G C S E P h y s i c s – A t o m i c S t r u c t u r e

( p a g e 2 )

Rutherford's famous alpha particle experiment

• Alpha particles were fired at gold foil

• Alpha particle have a positive charge as contain 2 protons and 2 neutrons and no electrons

Observation What it meant

Most of the alpha particles wert straight through the gold foil

This showed that the atom was mainly empty space (unlike the plum pudding where the model said it was a dense mass)

Some of the particles were deflected

This happened because the positive nucleus repelled the positive beta particle causing it be deflected. This showed that the positive mass held in the nucleus and not even spread out like the plum pudding model.

Very very few alpha particles reflected backwards

This showed that only a very small number of alpha particles hit the nucleus directly head on and relfected back, this showed that the nucleus was very very small

G C S E P h y s i c s – A t o m i c S t r u c t u r e

( p a g e 3 )

Alpha (α) radiation Beta (β) radiation

Gamma () radiation

Half Life the time taken for the number of radioactive nuclei in a sample to halve.

Sources of background radiation

Nuclear Equations

Irradiation This is where an object is exposed to radiation Most background radiation is harmless Some irradiation such as from X-Rays or irradiation of medical equipment to kill bacteria and sterilise it or used in the food industry to make sure the food does not spoil has such a short half life it is not harmful

Damaging cells Radiation causes the DNA to become ionised, this breaks and damages the DNA. The cell can die or it can undergo repair. If it is repaired accurately the cell functions normally. It can undergo misrepair which can lead to the cells developing into cancer.

What is radioactive contamination and why it is a hazard and why contamination from an alpha emitter is much more dangerous than the other sources of radiation: • It is unwanted presence of radioactive atoms. • From coming into contact through touching, ingesting or

breathing something in • Radioactive atoms can ionise cells causing damage to cell

DNA • This can result in mutations in the DNA and genes or

cancer • Alpha is the most ionising radiation and least penetrative • This means it gets stuck and ionises all the cells it comes

in to contact with

G C S E P h y s i c s – A t o m i c S t r u c t u r e

( p a g e 4 )

Brachytherapy This is where a small sealed radioactive source or seed is placed in to the tumour itself, to give a high dose of radiotherapy directly to the tumour but a much lower dose to the surrounding tissues. Brachytherapy is mainly mused to treat cancers in the prostate gland, cervix and womb. It is sometimes given in addition to radiotherapy.

Radiotherapy • 3 sources of radiation (X-ray) are

arranged around a patients head • Each carrying 1/3rd of the dose. • The healthy tissue it travels through

will also be destroyed or damaged • To prevent this the sources are

rotated • So the tumour receives constant

radiation • The surrounding tissues receive only

1/3rd but not all the time • X rays used as can control the amount

of energy given off

How can a radioactive tracer be used to diagnose a blocked blood vessel? • Measure the background radiation so you don’t take this into account for your experiment and know the actual

radiation emitted from the source • The radioisotope would be injected into the blood vessel • Using a Geiger counter we would measure the radiation from outside the body as it moved through the blood vessel • If the radiation stopped moving at a certain point this would indicate the location of the blockage • Gamma radiation is used as it is the least ionising to cells • This means it will cause the least damage to surrounding cells • Gamma is also the most penetrative so will be detected outside the body

What is peer review? When scientists were investigating nuclear radiation they were unaware on its effect on health and many died as a result. When the first atomic bombs exploded they did not understand the effects not on just immediate health but the health of generations of people and the effect long term on that area Peer review allows scientists to try to repeat others results and check the results

G C S E P h y s i c s – A t o m i c

S t r u c t u r e ( p a g e 5 )

Choosing a tracer All isotopes which are taken into the body must be gamma or beta emitters, so that the radiation passes out of the body. Alpha sources should never be used as they are highly ionising and do damage in a localised area. The source should only last a few hours too, so that the radioactivity inside the patient quickly disappears.

G C S E P h y s i c s – A t o m i c S t r u c t u r e –

Tr i p l e S c i e n c e ( p a g e 6 )

In order to do fusion you have to fuse 2 nuclei, both have a positive charge and repel each other. Very large amounts of energy are required to do this e.g. in the sun. This is how new elements are formed. The larger the 2 elements that are trying to fuse together the more energy required to over come the 2 positively charged nuclei.

1. Explain what is meant by ionising 2. Explain what is meant by penetrative 3. Explain the differences between alpha,

beta and gamma radiation 4. Explain the developments that led to the

current model of the atom 5. Explain Rutherford's famous experiment 6. Explain how his experiment disproved the

plum pudding experiment 7. Compare the plum pudding model to the

nuclear model 8. Explain what happens in alpha and beta

decay 9. U-238 (atomic number = 92) undergoes 2

alpha then 1 beta decay, write out the 3 nuclear equations to show the decay

10. Explain the purpose of peer review 11. Explain what is meant by radioactive

contamination 12. Explain what half life is 13. Explain how you can determine the half life

of a substance from a graph 14. Caron-14 has a half-life of 5700 years.

What fraction of its original activity will the sample have after 11 400 years?

15. How long will it take a sample of strontium-81 to decay to one eighth of its original value? (The half-life of strontium-81 is 22 minutes).

16. The activity of a neon-17 source is 1120Bq. What will it be after 0.5 seconds? (The half-life of neon-17 is 0.1 seconds).

17. The activity of an iridium-192 sample is 9600Bq. How long will it take to fall to 2400 decays per second? (The half-life of iridium-192 is 74 days).

18. An under floor fan is switched on to prevent any more radon-220 entering a house. The owner wants to know how long it will take for the radioactivity from the radon-220 in the house to fall to less than one thousandth of its original value. The half-life of radon-220 is one minute. How many half-lives will this take? How long will this take?

19. Explain how radiation can be used to treat cancer

20. Explain the difference between fission and fusion

21. Explain why large amounts of energy are required in a fusion reaction and why this is not possible on earth

22. Explain what a chain reaction is in a fission reaction

23. Explain how the chain reaction can be controlled in a nuclear reactor

G C S E P h y s i c s – A t o m i c S t r u c t u r e Q u e s t i o n s

G C S E P h y s i c s – A t o m i c S t r u c t u r e Q u e s t i o n s